A Baker cyst results from an accumulation of trapped joint fluid, which bulges from the joint capsule behind the knee as a protruding sac. Causes of the joint fluid accumulation include rheumatoid arthritis Rheumatoid Arthritis (RA) Rheumatoid arthritis is an inflammatory arthritis in which joints, usually including those of the hands and feet, are inflamed, resulting in swelling, pain, and often destruction of joints.... read more 
, osteoarthritis Osteoarthritis (OA) Osteoarthritis is a chronic disorder that causes damage to the cartilage and surrounding tissues and is characterized by pain, stiffness, and loss of function. Arthritis due to damage of joint... read more 
, other inflammatory joint diseases and overuse of the knees. Baker cysts often cause discomfort at the back of the knee but often do not cause symptoms. The cysts may enlarge to the size of a baseball and extend downward into the calf muscles.
A rapid increase in the amount and pressure of fluid within the cyst can cause it to rupture. The fluid released from the cyst can cause the surrounding tissues to become inflamed, resulting in symptoms that may mimic those of a blood clot in the calf (deep vein thrombosis [DVT] Deep Vein Thrombosis (DVT) Deep vein thrombosis is the formation of blood clots (thrombi) in the deep veins, usually in the legs. Blood clots may form in veins if the vein is injured, a disorder causes the blood to clot... read more 
). Moreover, a bulging or ruptured Baker cyst can rarely actually cause thrombophlebitis in the popliteal vein (which is located behind the knee) by pressing on the vein.
Diagnosis of Baker Cysts
A doctor's evaluation
Sometimes imaging tests
The doctor can usually make the diagnosis of a Baker cyst by asking the person specific questions about symptoms and feeling a swelling behind the knee or in the calf.
If necessary, ultrasonography Ultrasonography Ultrasonography uses high-frequency sound (ultrasound) waves to produce images of internal organs and other tissues. A device called a transducer converts electrical current into sound waves... read more 
, magnetic resonance imaging Magnetic Resonance Imaging (MRI) In magnetic resonance imaging (MRI), a strong magnetic field and very high frequency radio waves are used to produce highly detailed images. MRI does not use x-rays and is usually very safe... read more 
(MRI), or arthrography X-rays A doctor can often diagnose a musculoskeletal disorder based on the history and the results of a physical examination. Laboratory tests, imaging tests, or other diagnostic procedures are sometimes... read more 
can aid in the diagnosis, distinguish the cyst from a blood clot in the deep veins (DVT Deep Vein Thrombosis (DVT) Deep vein thrombosis is the formation of blood clots (thrombi) in the deep veins, usually in the legs. Blood clots may form in veins if the vein is injured, a disorder causes the blood to clot... read more ), and document how far the cyst extends.
Treatment of Baker Cysts
Nonsteroidal anti-inflammatory drugs (NSAIDs)
Joint or cyst aspiration and corticosteroid injection
Sometimes surgical removal of the cyst
NSAIDs Nonsteroidal Anti-Inflammatory Drugs Pain relievers (analgesics) are the main drugs used to treat pain. Doctors choose a pain reliever based on the type and duration of pain and on the drug's likely benefits and risks. Most pain... read more (or another pain reliever if NSAIDs cannot be taken) are the initial treatment of choice for pain relief.
When arthritis causes chronic knee swelling, the doctor may need to remove the fluid with a needle (a procedure called joint aspiration Joint aspiration (arthrocentesis) A doctor can often diagnose a musculoskeletal disorder based on the history and the results of a physical examination. Laboratory tests, imaging tests, or other diagnostic procedures are sometimes... read more ) and inject a long-acting corticosteroid (such as triamcinolone acetonide) to reduce the size of the cyst or prevent the formation of a Baker cyst. The doctor may also aspirate and inject the cyst. Removing the cyst surgically is an alternative if other treatments are not effective.
Sometimes cysts rupture and the cyst fluid is reabsorbed by the body. If the cyst has ruptured, the pain is treated with an NSAID or another pain reliever. If the ruptured cyst causes thrombophlebitis in the popliteal vein, treatment is bed rest, elevation of the leg, warm compresses, and anticoagulants (such as warfarin).
